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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

488 Noble Metals (Chemistry)

corrosion and tarnish in the oral environment. Thus, these 2. Silver
fillings are less noble and less permanent than the gold-
Silver is used in many states as a prophylactic against oph-
based alloys just discussed.
thalmia by placing a 1% AgNO 3 solution in the eyes of
new born infants. Other applications include using a 10–
3. Platinum Group Metals 20% AgNO 3 solution or solid AgNO 3 to remove warts or
cauterize wounds. Silver sulfadiazene is effectively and
Platinum usage is decreasing for the same reason as for widely used in preventing infections in burn victims. Sil-
gold. Less expensive metals are being substituted for it ver metal itself has an oligodynamic effect, which makes
(e.g., palladium and other alloys). it lethal to bacteria, and the insolubility of the metal makes
Palladium is used to a greater extent than platinum. it nontoxic to higher life forms so it can be a useful
Alloys such as 30Pd–40Ag–30Au with indium or tin are disinfectant.
used in porcelain veneering because of their lower cost,
good strength, ability to handle porcelain baking at ap-
◦
proximately 1000 C, and the previously discussed ability 3. Platinum
to closely match the thermal expansion coefficients of the
alloy and the porcelain. Some alloys have as high as 88% The most widely known medical application of platinum
palladium with the remainder being predominately silver. is in chemotherapy. Its compounds, the best known being
The iridium and tin will oxidize to yield a surface ox- cisplatin, cis-dichloro diammineplatinum(II), have been
ide coating that readily forms bonds with the porcelain to successful in treating testicular and ovarian cancers both
give a good alloy–porcelain bond. Alloys of Pd–Ag–Cu alone and in combination with other drugs. Cisplatin has
are beneficial when age hardening is desired. also shown promise in the treatment of other genitouri-
Iridium and ruthenium are used as minor components, nary, head and neck tumors, some forms of lung cancer,
mainly in gold alloys, to reduce the grain size in castings. as well as various pediatric cancers. It is speculated that
Rhodium and osmium have not had significant use in this the platinum complex inhibits cell division by binding
the DNA or by making the cancer cell susceptible to the
area.
body’s immune system. While this treatment is success-
ful, these complexes, primarily cisplatins, are toxic to the
J. Medical and Biological Applications kidneys. The toxicity can be reduced by increasing urinary
output to dilute the platinum passing through the kidneys.
Noble metal applications fall into two classifications: met- Carboplatin, a likely commercial prospect, is less toxic
als and their alloys used in medical application because of than cisplatin. The platinum thymine blues, aside from
their lack of reactivity and metal compounds and radioiso- their antitumor activity, react specifically with DNA to as-
topes used because of the special reactions they undergo sist in the visualization of nuclear components. The com-
in biological systems. plexes cisplatin and cis-diamminediaquoplatinum(II) ni-
trate are active against Rous sarcoma and fowl pox viruses.
Zeise’s salt, K[(C 2 H 4 )PtCl 3 ] will detect Hepatitis B in
1. Gold
blood sera, erythematosis, and rheumatoid arthritis.
Gold-198 prepared by irradiation of natural gold is im- The metal and its alloys are commonly used in the med-
planted in granular form in cancerous tissue or infused ical field. Platinum and 90Pt–10Ir are used in catheters,
in colloidal form as a treatment for bladder cancer. Gold- electrodes in heart pacemakers, and body implant probes
195m is used in a similar manner. Gold compounds have to reduce pain. For the latter application, an electrode can
been used in the past to treat tuberculosis and are currently be implanted in the brain to stimulate excretion of endor-
being used to treat arthritis (e.g., Auranofin). Gold(I) thi- phins to relieve pain. An example of this is in the treat-
ols are water soluble and have been useful for rheuma- ment of arachnoiditis, a painful inflammatory condition
toid arthritis. Some Gold(I) chloride complexes have been of one of the spinal cord coverings. Platinum electrodes
found to suppress adjuvant arthritis in rats, through oral have also been used in exploratory work to assist sight and
administration, without harm to the kidneys. Gold treat- hearing. Alloys of Pt–Ir and Pt–Ru are used in hypodermic
ments in general can cause pain, insomnia, and anxiety, as needles and surgical instruments because of their sharp-
well as affect the bone marrow and produce stomatitis and ness and durability. Magnets made of Pt–Co are found in
histamine reactions. Work is being done to find less toxic medical applications such as assisting in the control of
−
−
derivatives. The gold complexes [Au(CN) 2 ] , [AuCl 4 ] , artificial eyes, retention of implant artificial dentures, sys-
−
and [AuI 4 ] are used to label enzymes and proteins in X- tems for delivery of magnetic emboli via a guided catheter
ray diffraction studies. Gold colloids are used are used as to specific cerebral arteries, and for removing foreign
a cytochemical marker. objects.